Some of the earliest airbags for use in vehicle occupant protection systems were inflated by pressurized gas alone. These were large size airbags, such as for the passenger side airbag or the larger driver's side airbags, and a substantial volume of inflation gas was required. In turn, the pressure vessel containing the inflation gas was also quite large and difficult to fit in the space available for occupant restraint systems within a vehicle. In some instances, remote pressure vessels with long delivery passages were provided, which limited the response time required to inflate the airbag.
More recent development of inflators for airbags has centered on pyrotechnic inflators and hybrid inflators, both of which utilize heat expansion of gas to achieve a sufficient volume of gas for inflating an airbag from a relatively compact inflator. In the case of the pyrotechnic inflator, the heat expansion is a result of the burning of the pyrotechnic, and in the case of the hybrid inflator, the heat expansion is a combination of the pyrotechnic action and heating of stored pressurized gas.
Hybrid inflators utilize their pyrotechnic material to release the stored gas in one of two ways. First, the pyrotechnic material may be released into the pressurized gas chamber in order to heat and expand the gas therein, causing a burst disk to rupture as a result of increased pressure, or second, the pyrotechnic material may have a projectile associated therewith which is driven through a burst disk to release the pressurized gas, which is also heated and expanded by the pyrotechnic material in the pressurized gas chamber
Both the pyrotechnic and hybrid inflators are more compact than a purely pressurized gas system, but also result in a greater delay time before the onset of inflation. These longer delay times are not necessarily a drawback when applied to passenger and large driver's side airbags, wherein the desired inflation time is in part a function of the passenger's distance from the airbag.
Side impact airbags are positioned for deployment between the side of a vehicle and the occupant. The distance between the side of the vehicle and the occupant is relatively short, on the order of less than one foot, as compared to the distance separating a driver from a steering wheel and the even greater distance separating a passenger from a vehicle dash. Further, the front of a vehicle extends forwardly a substantial distance from the occupants and is designed with crumple zones which, in the event of a crash, provide a delayed and diminished interaction between the occupants and the steering wheel or dash. The side of a vehicle projects beyond the occupant by only the thickness of the door, the windows or the side body panels, offering very little opportunity for crumple zone protection. Vehicle design encompasses side beams to protect the integrity of the passenger compartment, and this is helpful in preventing injuries from a collapse of the side of the vehicle into the occupant's space. These design parameters for side impact protection do, however, leave a need for cushioning the passenger against interaction with the vehicle side. Side impact airbags used for that purpose require fast response and a fully inflated condition, also known as a "hard" bag, in order to be effective. The inflation response time, cost, and complexity of the pyrotechnic and hybrid inflators have made them difficult to adapt to smaller airbags.